1. Field of the Invention
The present invention relates to a method of patterning polymeric organic light emitters onto the substrate of an organic light emitting diode (OLED) using electrochemical polymerization, and to the resulting OLED device.
2. Description of the Related Art
OLEDs are known, and have been used for various types of displays. FIG. 1 illustrates a general overview of a portion of an OLED 10. Device 10 has a cathode electrode 12 that is spaced from a transparent anode electrode 14 deposited on a transparent substrate 18 which is typically comprised of glass or transparent plastic. Although only the anode electrode 14 is illustrated as transparent to allow light ho to pass through, the cathode electrode 12 or both the anode and cathode electrodes 14 and 12 may be transparent. An organic light emitter 20, which is capable of electroluminescence, is sandwiched between the cathode and anode electrodes 12 and 14. An encapsulating layer 23 may then be deposited on top of cathode electrode 12 to protect device 10 from the environment.
The organic emitter 20 has been comprised of electroluminescent thin films of either small discrete molecules such as aluminum tris(8-hydroxyquinoline) (Alq3) or dye-doped Alq3, or certain polymers. Different organic emitters emit different color light. For example, the Alq3 molecule emits green light. The cathode electrode 12 is usually a low work function metal such as an alkaline earth metal or reactive metal alloy. Examples of cathode electrodes include calcium, magnesium/silver, and aluminum/lithium. Typically, the anode electrode 14 is a high work function thin film of transparent indium tin oxide (ITO). Other materials used may be polyaniline or fluorine-doped tin oxide. Semitransparent metal films have also been used, although they tend to be less transmissive at thicknesses that are suitably conductive for electrodes. The phrase xe2x80x9cwork functionxe2x80x9d refers to the energy difference, in electron volts (eV), between a free electron and an electron at the Fermi level of the material. The xe2x80x9cFermi levelxe2x80x9d is the energy level at which the probability that an energy state is occupied is equal to one-half.
Referring back to FIG. 1, forward biasing device 10 (placing a higher voltage on the anode electrode 14 than on cathode 12) causes current to flow through the organic emitter 20. This current flow enables the recombination of holes injected at the anode electrode 14 with electrons injected from the cathode electrode 12 within the organic emitter, generating light h"ugr" in all directions, that is, electroluminescence. The light transmitted out to the sides of the device is lost and the light that hits the cathode is reflected. The output light h"ugr" is transmitted through the transparent anode 14 and the substrate 18.
FIGS. 2A and 2B illustrate in detail the electrodes of the OLED shown in FIG. 1. FIG. 2A illustrates a well-known pre-patterned transparent substrate 18 having parallel rows of transparent anode electrodes 14 deposited thereon. FIG. 2B illustrates parallel columns of deposited cathode electrodes 12 on a substrate 18A. Although both the anode electrodes 14 of FIG. 2A and the cathode electrodes. 12 of FIG. 2B are illustrated as straight strip patterns, other patterns may also be used. If rows and columns are used, then the electrodes are normally oriented orthogonal to each other. The intersection of an anode electrode 14 with a cathode electrode 12 defines a single pixel. All of the pixels together form a matrix from which images can be formed by illuminating desired pixel patterns. Respective leads 14a and 12a in a conventional matrix-addressing scheme electronically address the anodes and cathodes.
There are two common methods for the deposition (or patterning) of organic light emitters onto a substrate with a pre-patterned electrode, such as substrate 18 with pre-patterned anode electrodes 14. Both methods are similar in that emitter 20 is deposited as a corresponding matrix of discrete pixel elements in registration with the pixels defined by the electrodes. The choice of deposition method however, will depend upon the type of organic emitter used.
As mentioned above, the two main groups of organic emitters in common use are discrete molecules and polymers. For discrete molecules such as Alq3 or dye-doped Alq3, the preferred technique is vapor deposition through a mask (commonly referred to as masking). Ink jet printing is commonly used for polymeric organic emitters. Both methods are well known.
FIG. 3A illustrates the deposition of organic emitters R, B, and G (Red, Blue, and Green) onto the electrodes 14 of substrate 18 using the masking method. Masking uses a metal plate 30 (illustrated in FIG. 3B) with patterned openings 31 which are commensurate in pattern and number with the specific pattern and number of pixels that are to be deposited; conventional OLED displays can have millions of pixels. For a full color spectrum, at least three different masks 30R, 30B, and 30G (shown in FIG. 3C) may be used to deposit three different organic emitters, each with its own color emission characteristic, onto different sets of electrodes 14. Discrete organic emitter molecules that emit the desired colors are deposited through the mask openings onto the desired pixel areas of the underlying electrodes 14. Red emitter is deposited through openings 31R in mask 30R, blue emitter through openings 31B in mask 30B, and green emitter through openings 31G in mask 30G. The masks are placed as close to the substrate 18 as possible without touching it, to avoid disrupting the organic emitters previously deposited on the electrodes 14.
A drawback of masking is that the deposition areas 33 (shown within dashed lines in FIG. 3A) tend to be larger than the actual mask openings 31. Because the mask 30 does not touch the substrate 18, molecules passing through its openings 31 are diffused sideways through the gap between the mask 30 and the substrate 18, and are deposited on areas beyond the boundaries of the openings 31. With this method, there is a lack of control over exactly where the individual organic emitter molecules are deposited. This imprecise deposition (or patterning) can create overlaps 34 of different organic emitter molecules having different color emission characteristics onto the same pixel.
To overcome this problem, efforts have been made to place the mask 30 as close to the substrate 18 as possible and to reduce the size of the mask openings 31. However, since some gap is still required between the mask 30 and substrate 18 to avoid damage, spreading of the organic emitter molecules can still occur. While reducing the size of the mask openings 31 reduces the spreading problem, it also reduces the amount of light generated while increasing the spacing between pixels. Another proposal is to reduce the number of pixels and increase their size. While this could reduce or eliminate overlapping depositions, it would also reduce the resolution of the device.
FIG. 3D is a magnified illustration of a pixel 35 (shown within dashed lines), defined by the intersection of the anode 14 and cathode 12 electrodes, which is deposited with an organic emitter using the masking method. Pure color emission might not be possible if there is an overlap 34 of two or more different molecules with different color emission characteristics on the pixel region 35.
FIG. 4A illustrates the deposition of polymeric organic emitters R, B, and G onto electrodes 14 of substrate 18 using the ink jet method. With ink jet printing the polymeric organic emitter with the desired color emission characteristic is first dissolved in a solvent such as xylene, and then dropped in a discrete non-continuous manner onto desired pixel locations on the electrodes 14. The emitter in the solvent is in liquid form, and therefore the emitter drops spread 37 upon contact with the electrodes 14. Drop thickness and the amount of spreading across the substrate 18 are difficult to control, producing colors with different emission characteristics from two adjacent pixels deposited with the identical polymer. FIG. 4B is a magnified illustration of pixels 40 and 41 (shown within the dashed lines), defined by the intersection of the anode 14 and cathode 12 electrodes, which are deposited with the identical polymeric organic emitter (R) using this method. The variation in both thickness and spreading of the deposited polymer (R) onto these two adjacent pixels 40 and 41 could generate light with two different color emission characteristics, such as intensities, resulting in poor resolution.
Both vapor deposition and ink jet printing lack direct, full control over the deposition of organic emitters. They produce imprecise, unpredictable patterns that result in poor resolution.
The present invention provides full and direct control over organic emitter deposition in an OLED device, eliminating imprecise and unpredictable patterning to achieve accurate control over both thickness and lateral dimensions. It does this by using electrochemical polymerization to direct organic emitter deposition onto desired areas of the electrode on the substrate.
The invention can be implemented with a substrate that is patterned with at least one electrode and submerged into an electrochemical bath or electrolyte. The electrolyte solution includes a solvent, a supporting electrolyte salt which conducts charge, and monomer molecules that function as a precursor to an electroluminescent polymer that is to be deposited onto desired selected electrodes. An electrochemically inert counterelectrode is placed in the solution bath parallel to and facing the electrode substrate. In response to a voltage differential applied across the counterelectrode and desired selected electrodes on the substrate, a polymer is deposited onto the entire full length of the selected electrodes, with little or no overlap, spreading, or thickness variations. Although the technique is used to deposit polymeric organic emitters primarily onto anode electrodes, it may also be used in principle for cathodic deposition.
The invention can be implemented with certain conducting polymer precursor monomers, such as aromatic and heteroaromatic compounds which oxidize at relatively low anodic potentials to form polymeric electrical conductors, which, when reduced to their neutral forms, are electroluminescent. Examples can include derivatives of pyrrole, thiophene, furan, carbazole, and some electron-rich aromatics.